Heavy Metals and Microorganisms: The Story of Chromium Erin Field MB433 Outline Chromium Sources Speciation Remediation Strategies Metal-Microbe Interaction Case Study: Hanford Site Questions to be Answered Why is chromium contamination such a problem? What remediation strategies are there? What are the concerns associated with these remediation processes? Where does it come from? •Chromate Plating and Steel manufacturing Groundwater •Wood Preservatives (CCA) •Tanning Processes •Refractory bricks •Manufacturing dyes and pigments Chromium Speciation Chromium can be in many different forms ranging from Cr0 to Cr6+ Most commonly found as Cr(VI) and Cr(III) in the environment Chromium Speciation cont. • Hexavalent • Trivalent Chromium Chromium Cr(VI) Cr(III) •CrO42-, Cr2O72-, • Cr(OH)2+, H2CrO4, HCrO4- Cr(OH)2-, Cr(OH)30 Highly soluble Less soluble Highly mobile Less mobile Very toxic (known Less toxic carcinogen) Contamination Around the Country Estimated $360 billion in the United States for cleanup and prevention of metal contamination as of 2005 (Atlas and Philip 2005). Department of Energy Sites As of 2005, over 50% of the 170 DOE sites are contaminated with Chromium (Atlas and Philip 2005). Remediation Strategies Chemical Approaches (often changing pH with reducing agents) Quick Response Expensive to Apply Lack specificity to contaminant “Pump and Treat”: remove metals from a site in the aqueous phase and treated ex situ Expensive, inefficient, contaminants often higher than EPA standards “Dig and Dump”: dig up contaminated soil and dump it somewhere else Expensive, impractical for large sites Remediation Strategies Cont. Biological Approaches (using metal-microbe interactions to immobilize and decrease toxicity of metals) Inexpensive Can target specific metal contaminants Less impact on the ecosystem Can be used in situ and ex situ Metal-Microbe Interactions Metabolism-independent sorption including both adsorption and absorption by live or dead cells. Ligands involved include carboxyl, amine, hydroxyl, phosphate, and sulfhydryl groups. Molecular biologists are working on modifying the binding ligand to increase sorption of a specific metal. For instance, ZnO-binding peptides fused to fimbrae on the surface of E.coli. Negative influence on metal mobility. Organic acids (such as those produced through fermentation) can create a metal-chelate complex increasing the metal’s solubility and thus its mobility. Enzyme-mediated transformation of toxic metals to their less toxic forms usually through the use of these metals as electron acceptors. A cheap and less invasive method of bioremediation. Metals precipitate with enzymatically produced ligands such as sulfides and phosphates. For example, Citrobacter often creates phosphate-metal minerals. Exciting area for future research. Cr(VI) and Sulfate-reducers SO4- can be reduced to S2- which can create metal sulfide precipitates Cr(VI) can be actively transported into the cell through the sulfate transport system where it can damage DNA and indirectly generate oxygen radicals. Factors Influencing Cr(VI) Reduction Biomass Density Initial Cr(VI) Concentration Carbon Source pH Temperature Dissolved Oxygen Competing Electron Acceptors Soil composition Cr(VI) Reducing Microorganisms DOE Hanford Site, Richland, WA Established in 1943 as part of the Manhattan Project to manufacture plutonium for nuclear weapons. During the cold war additional nuclear reactors were built, most along the Columbia River. Water from the river was used to cool the reactors and then discharged back in. All reactors were finally shut down by 1990, but the radioactive and heavy metal waste remains. Pilot Scale Study In August 2004, 30lbs of 13C-labled Hydrogen Release Compound (HRC) were injected into a groundwater well at the Hanford Site to reduce Cr(VI) to Cr(III) and Cr(III) precipitate out on soil particles The HRC will yield lactate and hydrogen which can be utilized as electron donors in order to reduce Cr(VI) 30 lbs 13C- labled HRC Injection Well Monitoring well 15 ft. Did it Work? Maximum microbial cell counts were reached 13-17 days after injection. Additional Data Redox potential and dissolved oxygen data also suggest that the microorganisms were stimulated and reducing conditions occurred. Problems at Hanford Geology (fractures) Proximity to the Columbia River and plume migration Oxidation of Cr(III) Conclusions Chromium is a metal used in many industrial processes and continues to be a major concern of soil and groundwater contamination. Metal-microbe interactions can be exploited for bioremediation purposes Unlike organic contaminants, reduced metals such as Cr(III) can be re-oxidized. This is a serious concern that must be addressed at remediated sites. References Atlas, R.M., and Philip, J. (ed) Bioremediation: Applied Microbial Solutions for Real-World Environmental Cleanup. Washington,D.C.: ASM Press, 2005. Chen, J.M and Hao, O.J. (1998) Microbial Cr(VI) reduction. Critical Reviews in Env Sci and Tech 28(3):219-251. Department of Energy Hanford Site (visited 2006) www.hanford.gov. Field Investigations of Lactate-Stimulated Bioreduction of Cr(VI) to Cr(III) at Hanford 100H (visited 2006) http://www.esd.lbl.gov/ERT/hanford100h/index.html. Lloyd, J.R. and Lovley, D.R. (2001) Microbial detoxification of metals and radionuclides. Current Opinion in Biotechnology 12:248-253. Palmer, C.D. and Wittbrodt, P.R. (1991) Processes affecting the remediation of chromium-contaminated sites. Environ Health Perspect 92:25-40. Riley, R.G. and Zachara, J.M. (1992) Chemical contaminants on DOE lands and selection of contaminant mixtures for subsurface science research. Technical Report.
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